[RewriteStatepointsForGC] Fix an incorrect assertion

The assertion verifying that a newly computed value matches what is
already cached used stripPointerCasts() to strip bitcasts, however the
values can be not only pointers, but also vectors of pointers. That is
problematic because stripPointerCasts() doesn't handle vectors of
pointers. This patch introduces an ad-hoc utility function to strip all
bitcasts regardless of the value type.

Reviewed By: skatkov, reames

Differential Revision: https://reviews.llvm.org/D119994